CPP Device with Improved Current Confining Structure and Process

ABSTRACT

Plasma nitridation, in place of plasma oxidation, is used for the formation of a CCP layer. Al, Mg, Hf, etc. all form insulating nitrides under these conditions. Maintaining the structure at a temperature of at least 150° C. during plasma nitridation and/or performing post annealing at a temperature of 220° C. or higher, ensures that no copper nitride can form. Additionally, unintended oxidation by molecular oxygen of the exposed magnetic layers (mainly the pinned and free layers) is also avoided

This is a divisional application of U.S. patent application Ser. No.11/895719, filed on Aug. 27, 2007, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

This application is related to application Ser. No. 11/704,399, filedFeb. 9, 2007, herein incorporated, by reference, in its entirety.

FIELD OF THE INVENTION

The invention relates to the general field of CCP CPP GMR devices withparticular reference to the composition and formation of the currentconfining layer.

BACKGROUND OF THE INVENTION

Current perpendicular to plane (CPP) giant magnetoresistance (GMR) basedmagnetic heads are considered as promising candidates for achieving arecording density of over 200 Gb/in² [1-2]. In a CPP GMR head structurea bottom synthetic spin valve type film stack is employed for biasingreasons, and a CoFe/NiFe composite free layer is conventionally usedfollowing the experience with Current in Plane (CIP) GMR devices. It hasalready been demonstrated [3] that by laminating CoFe AP1 layers withthin Cu, CPP GMR performance can be improved. An example of a metallicCPP spin valve structure is:

Seed/AFM/AP2/Ru/[CoFeCu]/Cu30/[CoFe/NiFe]/cap

It has also been proposed [4] that CPP GMR performance will be furtherimproved by a current confining path (CCP) in the Cu spacer achieved byproviding a segregated metal path within an oxide. An example of aCCP-CPP spin valve structure is as follows:

Seed/AFM/AP2/Ru/[CoFeCu]/Cu/CCP-layer/Cu/[CoFe/NiFe]/cap

In a typical CPP spin valve structure of either case shown above, theAP1 or AP2 thickness is in the range of 20-50 Å, and the free layerthickness is in the range of 30-60 Å. For read head applications, thefree layer is preferred to have a small coercivity (Hc) of less than 10Oe and a low magnetostriction in the order of E-8 to low E-6 to reducethe stress-induced anisotropy.

For the past few years, there has been a lot of progress in thedevelopment of either metal CPP or CCP-CPP heads. However, for the metalCPP case, the CPP GMR ratio remains at a rather low value; for CCP-CPPschemes, since the current path is confined to Cu metal channels thatare connected through an oxide matrix (AlOx, TiO₂, or MgO layers) [5],the Cu purity is easily compromised during the insulator formationprocess, making the latter critical to ensuring a high MR ratio. Aschematic cross-section of a CCP layer is shown in FIG. 1. Conductionthrough copper layer 11 can be seen to continue upward through copperfilaments 12 which are surrounded by insulating oxide regions 13. Thepresent invention discloses a method and structure in which nodegradation of the Cu purity and the MR ratio will occur.

-   [1] M. Lederman et al U.S. Pat. No. 5,627,704-   [2] J. W. Dykes et al U.S. Pat. No. 5,668,688-   [3] Toshiba TMRC 2001-   [5] M. Li et al, U.S. 2006/0007605

A routine search of the prior art was performed with the followingreferences of interest being found:

In U.S. Pat. No. 7,005,691, Odagawa et al. show a non-magnetic copperfilm and an insulating film comprising an oxide, carbide, or nitride. InU.S. Patent Application 2005/0157433, Kamiguchi et al. show a resistanceregulating layer in a copper non-magnetic layer comprising an oxide,nitride, fluoride, carbide, or boride.

U.S. Patent Application 2005/0094322 (Fukuzawa et al.) teaches plasmaoxidation or nitridation of a metal non-magnetic layer. U.S. PatentApplication 2005/0002126 (Fujiwara et al) says “instead of oxidation,nitridation may work if materials with different susceptibility tonitridation are chosen.” However, this notion is not pursued anyfurther, no detailed process or explanation being given. Additionally,the basic process underlying this invention is significantly differentfrom the present invention. For example, the locations of the CCP layersand how to form them.

SUMMARY OF THE INVENTION

It has been an object of at least one embodiment of the presentinvention to provide a Confined Current Path structure in which thecopper conducting filaments are free of oxygen contamination.

Another object of at least one embodiment of the present invention hasbeen that said structure, when used in the context of a GMR or MTJdevice, have associated magnetic layers, such as the pinned and freelayers, that are free of oxygen contamination.

Still another object of at least one embodiment of the present inventionhas been to provide a process for forming said CCP structure.

A further object of at least one embodiment of the present invention hasbeen that said process be easily adaptable for incorporation as part ofprocesses currently in use for the manufacture of GMR and MTJ devices.

These objects have been achieved by the use of plasma nitridation, inplace of plasma oxidation, for the formation of the CCP layer. Al, Mg,Hf, etc. all form insulating nitrides under these conditions. Bymaintaining the structure at a temperature of at least 150° C. duringthe plasma nitridation we ensure that no copper nitride can form.Additionally, unintended oxidation by molecular oxygen of the exposedmagnetic layers (mainly the pinned and free layers) is also avoided. Thenet result is better dR/R and better overall performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a CCP layer of the prior art.

FIG. 2 shows a CCP layer according to the present invention wherein thecopper filaments are embedded in a matrix of an insulating metallicnitride.

FIG. 3 shows how the structure of FIG. 2 may me repeated in order toincrease the lengths of the copper filaments.

FIG. 4 illustrates the present invention as used as part of a GMR (orMTJ) device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In conventional CPP devices, a Cu spacer is used either as a full filmlayer for metal CPP or as a confined metal path in the CCP-CPP scheme.In the metal CPP case, since the resistance of Cu is very small, the CPPdR/R is shunted away to a small value. In the CCP-CPP case, the Cu metalpath is formed and confined within an insulating template, typicallyAlOx or MgO, so that ARA can be enhanced quite significantly.

Insulator formation is generally accomplished using ion-assistedoxidation (IAO), such as plasma oxidation, radical oxidation, ozoneoxidation or even natural oxidation. It is critical to preserve thecopper purity during this step if one is to achieve the best possible MRratio. Thus, the insulator formation process should result only in theformation of the insulator layer without oxidizing the Cu metal path orany other critical layers such as the free and AP1 (pinned reference)layers. In the prior art, during the oxidation of Al, Mg or the likematerials, although Cu is more inert to oxidation than Al or Mg, it isinevitably oxidized to some extent during the energized oxidationprocess, especially when the Cu layer is very thin as it is for CCP-CPPscheme. This inevitably leads to undesirable Cu purity degradation andhence a lower dR/R.

As illustrated in FIG. 2, the present invention overcomes this problemby forming the insulator material in which the copper filaments 12 areembedded from metallic nitride 23 rather than an oxide. It is well knowthat Cu is more inert than Al. However, in a plasma environment copperstill reacts with ionized oxygen to form one or more of several possiblecopper oxides. This is because the oxygen is too active to selectivelyoxidize the Al but not the Cu, resulting in a net deterioration of theCu purity. The stronger the oxidation process, the more the Cu layergets oxidized and therefore the lower the dR/R.

Although nitrides of aluminum (and similar metals such as Hafnium orMagnesium) are readily formed using plasma nitridation, copper nitrideis unstable and will dissociate, starting at temperatures as low asabout 100° C., so that copper, even when exposed to active nitrogen,cannot be nitrided if it is at a temperature of about 150° C., orhigher, when it is exposed to the nitriding environment. In fact, thepurity of the copper will generally be further enhanced through theremoval of surface copper oxide during the nitridation process.

Following current practice for magnetic head manufacturing, there is apost annealing treatment at a temperature of 220° C. or higher for aperiod of 2 hours or longer. So, even if some trace of CuN remains afterthe plasma nitridation, it will decompose to Cu during thispost-annealing.

An additional benefit to the use of plasma nitridation in place ofplasma oxidation is that the possibility of degrading the magneticproperties of the free layer and/or the AP1 layer is much less. Duringthe plasma oxidation process excess (unactivated molecular) oxygen canbe captured by the neighboring AP1 and free layer since these layersinclude iron which is an easy attractor to oxygen. On the other hand,when the plasma nitridation process is used, after the insulating layeris formed, the excess nitrogen will not affect other layers sincenitrogen is inert outside the plasma environment.

In order to increase the length of the copper filaments that make up aCCP layer, the process of laying down a copper layer, followed by anitridable metallic layer and then repeating the plasma nitridation stepmay be repeated one or more times. The result, as schematicallyillustrated in FIG. 3, is the continuation of the original filaments 12as filaments 32

Thus, with no changes to the preceding and succeeding steps, one cansimply use plasma nitridation in place of the oxidation process. Theresulting structure then looks as follows.

Seed/AFM/AP2/Ru/[CoFe_Cu]/Cu/CCP-layer/Cu/[CoFe/NiFe]/cap

In the above configuration, as in the prior art, a bottom electrode,Ta/Ru, is typically used as the seed layer and IrMn as theantiferromagnetic pinning layer. In the synthetic AP structure, FCC-likeFe10%Co /Fe70%Co /Fe10%Co is typically used as the AP2 while theFe70%Co, laminated with Cu, is used as the AP1 (pinned reference) layer,shown as layer 42 in FIG. 4. In FIG. 4, element 41 represents all layersbelow AP1 (Ru, AP2, AFM, seed, etc.) while layer 43 is equivalent tolayer 11 of FIG. 2. Layer 44 is the copper spacer layer portion of theGMR structure which would be replaced by a tunneling insulation layer ifthis were a Magnetic Tunnel Junction (MTJ) device. The free layergenerally comprises CoFe/NiFe and Cu/Ru/Ta/Ru is applied as cappinglayer 45 onto which is deposited top electrode 46.

As disclosed in the present invention, the CCP layer is formed throughplasma nitridation of Al, AlCu, Mg, AlMg, Hf, Ta, Cr, Ti, Si and Zr orthe like. For example, Al, AlCu, Mg or AlMg (3-20 A)/ PT(10W˜200 W, 10sccm˜200 sccm Ar, up to ˜200 sec)/plasma nitridation (about 10 to about300 W, about 10 to about 200 sccm Ar with about 0.01˜50 sccm N₂, 5sec˜1000 sec).

In summary, the use of plasma nitridation for the formation of the CCPinsulating layer improves the purity of the Cu layer and of theneighboring magnetic layers, thereby offering the following advantages:

-   (a) Greater Cu purity.-   (b) Prevention of associated magnetic layer oxidation.-   (c) Improved dR/R.

What is claimed is:
 1. A process to form a CCP CPP GMR device,comprising: providing a magnetically pinned layer on a substrate;depositing a layer of copper on said pinned layer; depositing, on saidcopper layer, a layer of a nitridable material mixed with an amount ofcopper; then, while maintaining said copper layer at a temperature of atleast 150 deg. C whereby said copper layer cannot be converted to coppernitride, subjecting said layer of nitridable material to a nitridationprocess, thereby forming a current confining layer without oxidizingsaid pinned layer; depositing a magnetically free layer on said currentconfining layer whereby no unintended oxidation of said free layer canoccur; and depositing a top electrode on said free layer.
 2. The processof claim 1 further comprising depositing one or more additional layersof nitridable material, mixed with copper, on said current confininglayer and subjecting each of said additional layers to said nitridationprocess whereby a thicker current confining layer is formed.
 3. Theprocess of claim 1 wherein said amount of copper mixed with saidnitridable material ranges from 0 atomic % to about 20 atomic %.
 4. Theprocess of claim 1 wherein said nitridation process further comprises aplasma treatment (10-200 W, at about 10-200 sccm, Ar or Kr, for up toabout 200 sec) of the surface of the Al or other nitridable material toa depth of about 3 to 20 Angstroms, followed by plasma nitridation withRF power of about 10 to 300 W at 10 to 200 sccm in Ar or Kr mixed with0.01 to 50 sccm of nitrogen for a period of from 5 sec to 1000 sec. 5.The process of claim 1 wherein said nitridable material is selected fromthe group consisting of Al, Mg, AlMg, Hf, Ta, Cr, Ti, Zr and Si etc 6.The process of claim 1 wherein said copper layer is deposited to athickness of up to about 15 angstroms and said nitridable layer isdeposited to a thickness of between about 3 and 20 angstroms.